In a fuel injection system for injecting fuel into an internal-combustion engine the fuel injection valves of individual cylinders are frequently coupled to a common pressure accumulator for obtaining fuel therefrom.
Each fuel injection valve is provided with a valve needle (nozzle needle) which is coupled to a controllable setting drive and which thus closes (blocks) or opens (unblocks) the fuel outlet opening of the nozzle. During operation, the valve needle is exposed to a liquid pressure of, for example, 1500 bar so that for achieving short and reproducible control periods in the opening and closing processes, between the valve needle and the setting drive conventionally a pressure compensating piston is arranged for moving the valve needle with a reduced pressure. The pressure equalizing piston is, dependent on the switching position of the control valve, coupled either with the pressure accumulator or with a fuel return (discharge) conduit. In this arrangement care has to be taken that the control valve is so designed that it is pressure balanced relative to the pressure accumulator, the fuel return conduit and the pressure equalizing piston.
Conventional valve arrangements of the above-outlined type have the disadvantage that during the switching process the control valve briefly assumes an intermediate position, so that the high-pressure side is directly in communication with the low-pressure side of the system. This leads, in phase with the switching motion, to appreciable pressure drops in the pressure accumulator. Since in a multi-cylinder engine a plurality of fuel injection valves are present, in the pressure accumulator pressure fluctuations occur at the frequency of the cycling sequence of all the fuel injection valves. Such oscillation phenomena in the application of pressure, however, affect the switching behavior of the valves and may lead to non-reproducible magnitudes in the periods of injection and the quantities of the injected fuel at the individual valves.